C# Async Multiple Web Requests to DataTable – Work appears to stop for unknown reasons – Ebay API

I am developing a single form app that, on button press, makes a simple inventory database API query, and checks each returned ItemID# for a corresponding image which may or may not exist at a URL formed from the ID#. I am currently doing this by sending HttpWebRequest.Method = “HEAD” requests for each URL, returning true unless the catch block is triggered.

The database query may return 50 – 150 part numbers, and sending HEAD requests to each one individually in this way takes about 5 minutes and this is not productive.

I am trying to multi-task this process using async and await. When I click the button, it works fine, asynchronously loading rows into my DataGridView one by one, at a rate of about 2/second (which isnt bad, but I would still like to speed this up if possible).

HOWEVER: After finding 2 successful URL responses, it stops loading rows and appears to just give up, for reasons unknown to me??? And the syncContext block which re-enables the UI is never executed, because the work is never completed. Can anyone see what might be causing this to happen?

I have been working loosely based off this doc:

“How to: Make Multiple Web Requests in Parallel by Using async and await (C#)”
https://msdn.microsoft.com/en-us/library/mt674880.aspx

namespace ImageTableTest
{
public partial class ImageTableTestForm : Form
{
    //P21 Authentication Variables
    private static Token P21token = null;
    private static RestClientSecurity rcs;

    //Create Tables and bindingSource
    DataTable itemDataIMG = new DataTable();
    DataTable itemDataNOIMG = new DataTable();
    DataTable itemDataComplete = new DataTable();
    BindingSource bindingSource = new BindingSource();

    private readonly SynchronizationContext synchronizationContext;


    public ImageTableTestForm()
    {
        InitializeComponent();

        //Create syncContexct on UI thread for updating UI
        synchronizationContext = SynchronizationContext.Current;

        //authenticate database API function
        authenticateP21();     

        //Designing DataTables
        itemDataIMG.Columns.Add("MPN#", typeof(string));
        itemDataIMG.Columns.Add("IMG", typeof(bool));
        itemDataIMG.Columns[1].ReadOnly = true;

        itemDataNOIMG.Columns.Add("MPN#", typeof(string));
        itemDataNOIMG.Columns.Add("IMG", typeof(bool));
        itemDataNOIMG.Columns[1].ReadOnly = true;

        itemDataComplete.Columns.Add("MPN#", typeof(string));
        itemDataComplete.Columns.Add("IMG", typeof(bool));
        itemDataComplete.Columns[1].ReadOnly = true; 

        //bind to DataGridView itemView
        bindingSource.DataSource = itemDataComplete;          
        itemView.DataSource = bindingSource;
        itemView.AutoGenerateColumns = false;
    }



    private async void testBtn_Click(object sender, EventArgs e)
    {
        //When button is clicked, disable UI and
        //start background work:
        testBtn.Enabled = false;
        loadSpinner.Visible = true;

        await Task.Run(() =>
        {
            getItemView();
        });
    }


    private async void getItemView()
    {
        try
        {
            //This executes the query and returns an array of Part objects:
            PartResourceClient prc = new PartResourceClient(ConfigurationManager.AppSettings["P21.BaseURI"], rcs);
            prc.QueryFilter("add_to_ebay eq 'Y'");
            Part[] pResults = prc.Resource.GetParts();              

            int numParts = pResults.Length;                
            Task<bool>[] taskArray = new Task<bool>[numParts];
            bool[] IMGboolArray = new bool[numParts];

            //For each part, create CheckImageURL task and add to task Array
            //Then Await execution
            for (int i = 0; i < numParts; i++)
            {
                taskArray[i] = CheckImageURL(pResults[i].ItemId);
                IMGboolArray[i] = await taskArray[i];
            }                
        }
        catch (Exception e)
        {
            MessageBox.Show(e.ToString());
        }

        //When all Tasks finish, remove loadSpinner, re-enable UI
        //(This never executes for unknown reasons.)
        synchronizationContext.Post(new SendOrPostCallback(o =>
        {              
            loadSpinner.Visible = false;
            testBtn.Enabled = true;
        }), null);

        MessageBox.Show("<DONE>");
    }


    async Task<bool> CheckImageURL(string MPN)
    {
        //Here I am forming and executing the web HEAD request,
        //If there is there is a 'NOT FOUND' response it goes to 'catch' block:
        string URL = "https://s3-us-west-2.amazonaws.com/www.crosscreektractor.com/ebay-images/" + MPN + "_e.png";
        HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(URL);
        request.Method = "HEAD";
        try
        {
            await request.GetResponseAsync();
            synchronizationContext.Post(new SendOrPostCallback(o =>
            {
                addDataRows(MPN, true);
            }), null);

            return true;
        }
        catch
        {
            synchronizationContext.Post(new SendOrPostCallback(o =>
            {
                addDataRows(MPN, false);
            }), null);

            return false;
        }
    }

    private void addDataRows(string MPN, bool IMG)
    {
        //Add data to respective table:
        if (IMG)
        {
            itemDataIMG.Rows.Add(MPN, IMG);
        }
        else
        {
            itemDataNOIMG.Rows.Add(MPN, IMG);
        }

        //Here I am sorting the IMG and NOIMG tables,
        //then merging them into the Complete table which
        //The DataGridView is bound to, so that IMG entries are on top:
        itemDataIMG.DefaultView.Sort = ("MPN# DESC");
        itemDataNOIMG.DefaultView.Sort = ("MPN# DESC");

        itemDataComplete.Clear();
        itemDataComplete.Merge(itemDataIMG);
        itemDataComplete.Merge(itemDataNOIMG);
        itemView.Refresh();
    }